1. Field of the Invention
This invention relates generally to oil well completion strings and more particularly to a hydrostatically-balanced open hole gravel pack system wherein hydrostatic pressure is maintained on the formation throughout the gravel packing operations.
2. Description of the Art
To obtain hydrocarbons from earth's subsurface formations, wellbores or boreholes are drilled into hydrocarbon-bearing formations or producing zones. After drilling a wellbore to the desired depth, a completion string containing various completion and production devices is installed in the wellbore to produce the hydrocarbons from the production zone to the surface. In one method, a fluid flow restriction device, usually containing one or more serially connected screens, is placed adjacent the production zone. Gravel is then packed in the space or annulus between the wellbore and the screen. No casing is installed between the screens and the wellbore. Such completions are called "open hole" completions and the systems used to gravel pack are called open hole gravel pack systems.
In commercially used open hole gravel packing system a completion string is frequently utilized for gravel packing. The completion string usually includes a screen near its bottom (or the downhole end), at least one packer or packing element above the screens, and a mechanism above the packer that allows gravel slurry to flow it from the surface to the annulus between the screens and the wellbore, and the clean fluid to return from the completion string to the surface. To gravel pack the annulus between the formation and the completion string, packer is set to form a seal between the completion string and the wellbore, the packer prevents the hydrostatic pressure from being applied to the formation, which prevents, for a period of time, maintaining the hydrostatic pressure above the formation pressure (the "overbalanced condition" or "overburdened condition") during the gravel pack operation. Thus, the formation pressure can exceed the hydrostatic pressure, which can cause hole damage or well collapse and damage to the filter cake.
A substantial number of currently drilled wellbores are highly deviated or horizontal. The horizontal wellbores are extremely susceptible to damage if the overbalanced conditions are not maintained throughout the gravel pack operations or during any other completion operation. Maintaining the wellbore under overbalanced condition throughout the gravel packing, especially in highly deviated and horizontal wells is very desirable. The present invention provides a gravel pack system and method which maintains the pressure on the formation above the formation pressure throughout the gravel packing operation. The present system also is simpler and easier to use, thereby reducing the overall completion or gravel pack operations time and cost.